If you are downloading the app from legit stores [Apple store, Google Play], then it is fine. Adidas is indeed running a campain to get more users download the app and get exclusive shoes that they don't intend to sell in retail market.
The adidas strategy seems to make the product exclusive and available only to individuals. Note you have to be in person and show photo ID at the adidas retail outlet, pay and pick-up the shoes. One can only register once per phone number. The reservation is random, first come first reserved.
The person on facebook is trying to circumvent this by having quite a few people download the app and book. He is then looking at buying this from you. Not sure what would the price of shoes be and is the 350 EUR in addition to the price of the shoes that you need to pay the store.
I couldn't find about Paris, but there were similar campaigns in US last December.
WHY DO I HAVE TO REGISTER IN THE CONFIRMED APP? In order to have a
chance to get a reservation through adidas Confirmed you need to sign
up in the app. If you are able to get a reservation, we will use this
information (plus your photo ID) to verify your identity when you pick
WHAT ARE THE STEPS TO MAKE A RESERVATION?
STEP 1: Create an account in the app, verify through SMS, and enable location services and push notifications.
STEP 2: Follow @adidasoriginals on Twitter to learn when reservations open.
STEP 3: Once the reservation period begins, open the app and navigate to a product page to select your size and confirm your reservation. You must be located within New York City, Chicago, or Los Angeles areas to participate.
STEP 4: If you get a reservation within adidas Confirmed you will receive a retail location and timeframe for pickup within the app.
STEP 5: Go to the designated location to complete purchase and receive product.